The present invention relates to the long-term storage of spent fuel that has been removed from a nuclear reactor, and more particularly, to a closure system which can be removeably applied to a spent fuel storage cask during development, testing, and demonstration of the cask and which can also be used to permanently seal the cask during long-term storage, after the development, testing, and demonstration have been completed.
FIG. 1 illustrates a typical fuel assembly 20 for supplying nuclear fuel to a reactor. Assembly 20 includes a bottom nozzle 22 and a top nozzle 24, between which are disposed elongated fuel rods 26. Each fuel rod 26 includes a cylindrical housing made of a zirconium alloy such as commercially available "Zircalloy-4", and is filled with pellets of fissionable fuel enriched with U-235. Within the assembly of fuel rods 26, tubular guides (not shown) are disposed between nozzles 22 and 24 to accommodate movably mounted control rods (not illustrated) and measuring instruments (not illustrated). The ends of these tubular guides are attached to nozzles 22 and 24 to form a skeletal support for fuel rods 26, which are not permanently attached to nozzles 22 and 24. Grid members 28 have apertures through which fuel rods 26 and the tubular guides extend to bundle these elements together. Commercially available fuel assemblies for pressurized water reactors include between 179 and 264 fuel rods, depending upon the particular design. A typical fuel assembly is about 4.1 meters long, about 19.7 cm wide, and has a mass of about 585 kg., but it will be understood that the precise dimensions vary from one fuel assembly design to another.
After a service life of about three years in a pressurized water reactor, the U-235 enrichment of a fuel assembly 20 is depleted. Furthermore, a variety of fission products, having various half-lives, are present in rods 26. These fission products generate intense radioactivity and heat when assemblies 20 are removed from the reactor, and accordingly the assemblies 20 are moved to a pool containing boron salts dissolved in water for short-term storage. Such a pool is designated by reference number 30 in FIG. 2.
Pool 30 is typically 12.2 meters deep. A number of spent fuel racks 32 positioned at the bottom of pool 30 are provided with storage slots 34 to vertically accommodate fuel assemblies 20. A cask pad 36 is located at the bottom of pool 30.
During the period when fuel assemblies 20 are stored in pool 30, the composition of the spent fuel in rods 26 changes. Isotopes with short half-lives decay, and consequently the proportion of fission products having relatively long half-lives increases. Accordingly, the level of radioactivity and heat generated by a fuel assembly 20 decreases relatively rapidly for a period and eventually reaches a state wherein the heat and radioactivity decrease very slowly. Even at this reduced level, however, rods 26 must be reliably isolated from the environmnet for the indefinite future.
Dry storage casks provide one form of long-term storage for the spent fuel. After the heat generated by each fuel assembly 20 falls to a predetermined level--such as 0.5 to 1.0 kilowatt per assembly, after perhaps 10 years of storage in pool 30--an opened cask is lowered to pad 36. The cask typically contains a basket arrangement which provides a matrix of vertically oriented storage slots for receiving spent fuel. By remote control the spent fuel (either in the form of fuel assemblies 20 or in the form of consolidation canisters which contain fuel rods 26 that have been removed from fuel assemblies in order to increase storage density) is transferred to the basket arrangement in the cask, which is then sealed, drained, and flooded with a gas. The cask can then be removed from pool 30 and transported to an above-ground storage area for long-term storage.
The requirements which must be imposed on such a cask are rather severe. The cask must be immune from chemical attack during long-term storage. Furthermore, it must be sufficiently rugged mechanically to avoid even tiny ruptures or fractures during long-term storage and during transportation, when the cask might be subjected to rough treatment or accidents such as drops. Moreover, the cask must be able to transmit heat generated by the spent fuel to the environment while nevertheless shielding the environment from radiation generated by the spent fuel. The temperature of the rods 26 must be kept below a maximum temperature, such as 375.degree. C., to prevent deterioration of the zirconium alloy housing. The basket arrangement in the cask must be able to mechanically support the spent fuel under all realistic conditions while transferring heat generated by the spent fuel to the cask walls. Provisions must also be made to ensure that a chain reaction cannot be sustained within the cask before the water is drained. These requirements impose stringent demands upon the cask, which must fulfill its storage function in an utterly reliable manner.
In view of these demands it is not surprising that a considerable amount of development, testing, and refinement is warranted before a cask is ready for commercial production. It might be desirable to empirically confirm calculations concerning radiation levels or temperature, for example, or to test a new basket arrangement in actual practice. In a similar manner it might be desirable to check the internal condition of the cask or the fuel after a period of storage, or to test cask performance under different storage modes (i.e., intact fuel assemblies or consolidated fuel). In short, it will be apparent that it is desirable, during development, testing, and demonstration of a cask, to seal the cask with a removeable closure system in order to permit access to the cask interior. Nevertheless the object of this testing and refinement is to develop a cask which can be permanently sealed for a long-term storage of spent fuel. Moreover the closure system itself is part of what is tested; that is, it is desirable to test the cask using the closure system which will be used in actual practice.